This invention relates to electromagnetic flowmeters wherein excitation current for the electromagnetic coil is a periodic wave whose frequency is lower than the frequency of a commercial a-c power line.
In an electromagnetic flowmeter, the liquid whose flow rate is to be measured is conducted through the flow tube of a primary provided with a pair of diametrically-opposed electrodes, a magnetic field perpendicular to the longitudinal axis of the tube being established by an electromagnet. When the flowing liquid intersects this field, a voltage is induced therein which is transferred to the electrodes. This voltage, which is proportional to the average velocity of the liquid and hence to its average volumetric rate, is then amplified and processed in a converter or secondary to actuate a recorder or indicator.
The magnetic field may be either direct or alternating in nature; for in either event the amplitude of voltage induced in the liquid passing through the field will be a function of its flow rate. However, when operating with direct magnetic flux, the D-C signal current flowing through the liquid acts to polarize the electrodes, the magnitude of polarization being proportional to the time integral of the polarization current. With alternating magnetic flux operation, polarization is rendered negligible, for the resultant signal current is alternating and therefore its integral does not build up with time.
Though A-C operation is clearly advantageous in that polarization is obviated and the A-C flow-induced signal may be more easily amplified, it has distinct drawbacks. The use of an alternating flux introduces spurious voltages that are unrelated to flow rate and, if untreated, give rise to inaccurate indications. The two spurious voltages that are most troublesome are:
1. stray capacitance-coupled voltages from the coil of the electromagnet to the electrodes, and PA1 2. induced loop voltages in the input leads. The electrodes and leads in combination with the liquid extending therebetween constitute a loop in which is induced a voltage from the changing flux of the magnetic coil.
The spurious voltages from the first source may be minimized by electrostatic shielding and by low-frequency excitation of the magnet to cause the impedance of the stray coupling capacitance to be large. But the spurious voltage from the second source is much more difficult to suppress.
The spurious voltage resulting from the flux coupling into the signal leads is usually referred to as the quadrature voltage, for it is assumed to be 90.degree. out of phase with the A-C flow-induced voltage. Actual tests have indicated that this is not true in that a component exists in-phase with the flow-induced voltage. Hence, that portion of the "quadrature voltage" that is in-phase with the flow-induced voltage signal constitutes an undesirable signal that cannot readily be distinguished from the flow-induced signal, thereby producing a changing zero shift effect.
Pure "quadrature" voltage has heretofore been minimized by an electronic arrangement adapted to buck out this component, but elimination of its in-phase component has not been successful. Existing A-C operated electromagnet flowmeters are also known to vary their calibration as a function of temperature, fluid conductivity, pressure and other effects which can alter the spurious voltage both with respect to phase and magnitude.
Hence it becomes necessary periodically to manually re-zero the meter to correct for the effects on zero by the above-described phenomena.
All of the adverse effects encountered in A-C operation of electromagnetic flowmeters can be attributed to the rate of change of the flux field (d.phi.)/dt, serving to induce unwanted signals in the pick-up loop. If, therefore, the rate of change of the flux field could be reduced to zero value, then the magnitude of quadrature and of its in-phase component would become non-existent. Zero drift effects would disappear.
When the magnetic flux field is a steady state field, as, for example, with continuous D-C operation, the ideal condition d.phi./dt32 0 is satisfied. But, as previously noted, D-C operation to create a steady state field is not acceptable, for galvanic potentials are produced and polarization is encountered.
It is known with electromagnetic flowmeters to provide low-frequency excitation current to establish an alternating magnetic field, the excitation frequency being lower than the frequency of the commercial power line source so as to improve the zero-point stability of the flowmeter. Thus with a power line frequency of 60 cycles, the excitation frequency may be 15 Hz. The waveform of the excitation frequency may be sinusoidal, triangular, rectangular or in any other periodic form. The rectangular form is now widely used.
In the patent to Mannherz et al., U.S. Pat. No. b 3,783,687, whose entire disclosure is incorporated herein by reference, there is disclosed an electromagnetic flowmeter in which the excitation current for the electromagnetic coil is a low-frequency rectangular wave serving to produce a periodically-reversed steady state flux field, whereby unwanted in-phase and quadrature components are minimized without giving rise to polarization and galvanic effects.
In a low-frequency excitation system of the type disclosed in the Mannherz et al. patent, the electrode signal derived from the primary of the electromagnetic flowmeter is sampled in the secondary during intervals when the magnetic flux is substantially in a steady state condition. Since a drift of the output zero point depends largely on variations in the time differential of the applied magnetic flux, it becomes possible to stabilize the zero point by low-frequency excitation using rectangular magnetic flux and minimizing the time differential value of the magnetic flux by signal detection carried out by the above-noted sampling technique.
It has been found, however, that with an electromagnetic flowmeter having low-frequency excitation, when the distance between the primary of the flowmeter and the secondary thereof is relatively long, as, for example, longer than 100 meters, and when the conductivity of the fluid being metered is low, a transmission error arises due to capacitance encountered in the cable connecting the electrodes of the primary to the input of the secondary or converter. Even when shielded cables are employed, a span error of several percent occurs when the distance between the primary and secondary reaches about 300 meters. This error is attributable not only to stray capacity in the signal line but also to the capacity of the electrical double layer on each electrode. This value of this capacity depends on the electrode's area, its surface condition and the nature of the fluid being metered; hence it is not a constant value.